Mechanical Stress-Induced Defects in Thick a-PbO Layers.

Area of Science:

• Materials Science
• Medical Imaging Physics

Background:

• Amorphous lead oxide (a-PbO) is promising for direct conversion X-ray detectors due to low-temperature deposition and no signal lag.
• Commercial detectors use amorphous selenium (a-Se) with blocking layers to manage dark current, a technique applicable to a-PbO.

Purpose of the Study:

• To investigate the causes of stress-induced crystallization and defects in thick a-PbO layers.
• To propose a model explaining defect formation and suggest methods for improvement.

Main Methods:

• Development of a stress-induced crystallization model for a-PbO.
• Calculation of the thermal expansion coefficient of a-PbO.
• Analysis of deposition parameters' impact on heat accumulation and crystallization.

Main Results:

• Intrinsic stress in a-PbO layers increases with thickness, causing crystallographic defects (β-PbO phase).
• Thermomechanical mismatch due to thermal expansion is the primary stress source.
• Deposition parameters influence heat accumulation, promoting temperature-induced crystallization.

Conclusions:

• Stress-induced crystallization in thick a-PbO layers leads to defects, increased dark current, and cracking.
• Eliminating thermal expansion mismatches in blocking structures and controlling deposition heat are key to preventing defects.
• Optimized a-PbO layer growth can enable robust, thick films for advanced medical imaging detectors.